1. Field of the Invention
This invention relates to filtration and more particularly relates to filtration of dosing fluid in an exhaust aftertreatment system.
2. Description of the Related Art
Many internal combustion engines produce nitrogen oxides (NOx) as a by-product of combustion. Diesel engines in particular produce a relatively high amount of NOx in operation. Engines typically release NOx through the exhaust stream of the engine.
NOx is a pollutant that may aggravate asthmatic conditions, react with the oxygen in the air to produce ozone, and eventually form nitric acid when dissolved in water. Acid rain and smog are often attributed to the release of NOx from internal combustion engines.
Since the release of NOx is undesirable, various schemes have been implemented to reduce NOx emissions. One such scheme is a selective catalytic reduction (SCR) exhaust aftertreatment system. An SCR system reduces NOx emission using a chemical reaction between the exhaust gases, an additive, and a catalyst. A gaseous or liquid dosing fluid (most commonly ammonia or urea) is added to the exhaust gas and is absorbed onto a catalyst. The dosing fluid reacts with NOx in the exhaust gas to form harmless H2O (water vapour) and N2 (nitrogen gas).
In operation, an SCR system typically involves a tank of dosing fluid carried with the vehicle. The dosing fluid is pumped from the tank through an injector into the exhaust stream. One difficulty associated with SCR systems is contamination in the dosing fluid. This contamination can take the form of dirt or other foreign debris suspended in the dosing fluid, fragments worn off of elements of the SCR system, such as small pieces of an impeller from a pump, or other material. This contamination can restrict the flow of dosing fluid anywhere in the SCR system, but is particularly troublesome in the injector. Foreign material may lodge in the relatively small injector, reducing or eliminating the flow of the dosing fluid into the exhaust stream.
Some SCR systems include paper filters in housings between the tank of dosing fluid and the injector to reduce the incidence of contaminants in the injected dosing fluid. While these filters can improve the performance of the SCR system, they also introduce new difficulties. The paper filters have a capacity for holding contaminants that is low enough to require regular servicing over the life of the engine. Typically this involves replacing or cleaning the paper filter.
In addition, the dosing fluids used in SCR systems often have a relatively high freezing point. Urea, for example, has a freezing point of around −11 degrees Celsius. If a dosing fluid is allowed to settle in the filter housing in cold temperatures, the dosing fluid in the filter housing will freeze, eliminating the flow of dosing fluid and often damaging the SCR system. As a result, vehicles with SCR systems operating in cold temperatures often include heaters in the filter housings of the SCR system.
The existing approaches to filtering dosing fluids in SCR systems have high costs associated with both manufacturing and maintenance. Regular replacement of paper filter elements may require expensive service fees, and the addition of a heater element to a filter housing increases the cost to manufacture the system. These costs are ultimately borne by purchasers, owners, and operators of vehicles using these systems.